Electrostatic chucks are widely used in the semi-conductor manufacturing industry in order to temporarily hold silicon wafers in place while a variety of manufacturing processes are carried out on the wafers. To this end, such electrostatic chucks include a chucking electrode to create an electrostatic force on the upper surface of the electrostatic chuck.
In order to heat the held wafers during the manufacturing process, electrostatic chucks often include one or more embedded heating elements. In addition, it is very common for the electrostatic chuck to be carried by a metal or ceramic base that includes cooling channels formed therein, with the base acting as a heat sink. Coolant flows through the channels in the base in order to help control a temperature at the upper surface of the electrostatic chuck. The base, combined with the heating element embedded in the electrostatic chuck, make it possible to raise the temperature of the wafer to a predetermined temperature, and then lower the temperature of the wafer for further processing in a very rapid cycle.
It is also known to include gas passages in the upper surfaces of the electrostatic chuck in order to provide an inert gas to the backside of the wafer, which improves heat transfer between the electrostatic chuck and the wafer.
Electrostatic chucks, such as those described above, are currently fabricated using ceramic sintering technology in which the chucking electrode and heating element are embedded within layers of ceramic powder or pre-formed ceramic sheets, and then the overall structure is co-sintered to form a unitary body. The unitary body is then attached to the base using a bonding agent.
One of the issues with such conventionally formed ceramic electrostatic chucks is that the bonding agent, such as epoxy or silicone, that is used to bond the electrostatic chuck to the base does not hold up well to the severe plasma environment in which these electrostatic chucks are used. Consequently, the outer edge of the bonding agent is eroded over time, and thus the heat transfer characteristics of the bonding agent become different between the center and peripheral portions thereof. This, in turn, affects the uniformity of heat transfer at the upper surface of the electrostatic chuck and, consequently, along the upper surface of the wafer.
Another problem with organic bonding agents such as epoxy and silicone is that they are ineffective in mitigating the differences in coefficient of thermal expansion (CTE) between the ceramic electrostatic chuck and the underlying base, which is usually made of aluminum. As such, over time, delamination tends to occur along the bonding agent layer.
While it is also known to join the ceramic electrostatic chuck to the underlying metal base by brazing, the brazed materials that are currently used are very highly electrically and thermally conductive, which results in too much heat loss from the ceramic electrostatic chuck down to the base. This increased heat loss requires a longer amount of time between wafer-handling cycles and also requires more power to be supplied to the heating element to reach the target temperature on the upper surface of the wafer.
Another problem with ceramic electrostatic chucks regardless of how they are made is that, over time, polymer materials start to deposit on the side walls of the electrostatic chuck near the upper surface. This interferes with the ability of the wafer to sit flat on the upper surface of the electrostatic chuck, which in turn causes a high leak rate of the inert gas that is supplied to the back side of the wafer for temperature control. Eventually the polymer buildup becomes so great that the electrostatic chuck needs to be either completely refurbished or simply replaced. This takes a lot of time and is very expensive.